§ 349. EASTING OF THE TRADE-WINDS, ETC. 153 



Without other time-piece, the hour of the day may be told by 

 these maxima and minima, each group of which occurs twice a 

 day and at six-hour intervals. These invisible ebbings and flow- 

 ings — the diurnal change in the electrical tension — the diurnal 

 variation of the needle, and the diurnal rising and falling of the 

 barometer, follow each other as closely and as surely, if not quite 

 as regularly, as night the day. Any cause which produces changes 

 in atmospheric pressure invariably puts it in motion, giving rise 

 to gentle airs or furious gales, according to degree ; and here, at 

 least, we have a relation between the movements in the air and 

 the movements of the needle so close that it is difficult to say 

 which is cause, which effect, or whether the two be not the effects 

 of a common cause. 



349. Indeed, such is the nature of this imponderable called mag- 

 The question raised by Hctism, and such the suggcstious made by Fara- 

 modern researciiea. j^y's discovcrics, that the qucstiou has bccu raised 

 in the minds of the most profound philosophers of the age wheth- 

 er the various forces of light, heat, and gravitation, of chemical 

 affmity, electricity, and magnetism, may not yet be all traced to 

 one common source. Surely, then, it can not be considered as un- 

 philosophical to inquire of magnetism for some of the anomalous 

 movements that are observed in the atmosphere. These anoma- 

 lies are many; they are not confined to the easting of the trade- 

 winds ; they are to be found in the counter-trades and the calm 

 belts also. There is reason to believe, as has already been stated 

 (§ 288), that there is a crossing of the winds at the calm belts 

 (§ 212), and it was promised to go more into detail concerning the 

 circumstances which seem to favor this belief Our researches 

 have enabled us, for instance, to trace from the belt of calms, near 

 the tropic of Cancer, which extends entirely across the seas, an ef- 

 flux of air both to the north and to the south. From the sonth 

 side of this belt the air flows in a steady breeze, called the nortli- 

 east trade-winds, toward the equator (Plate I.) ; on the north side 

 of it, the prevailing winds come from it also, but they go toward 

 the northeast. They are the well-known westerly winds which 

 ])revail along the route from this country to England in the ratio 

 of two to one. But why should we suppose a crossing to take 

 place here? We suppose so from these facts : because throughout 

 Europe, the land upon which these westerly winds blow, prccipi- 



